Process of annealing and zinc coating ferrous articles



Jam 2 1934 PQTTER 1 9 512 121 PROCESS OF ANNEALING AND ZINQ COATING FERROUS ARTICLES Filed June 20, 1931 Patented Jan. 2, 1934 PATENT OFFICE PROCESS OF ANNEALING AND ZINC CQATIN G FERROUS ARTICLES William Harold Potter, Warrington, England Application June 20, 1931, Serial No 545,822, and

in Great Britain July 19, 1930 2 Claims.

This invention relates to the art of zinc coating ferrous articles, principally coiled wire and strip, and provides a method of annealing, and a method of chemical heat treatment, which produces 5 a zinc coated article such as wire or strip, in a continuous process, such zinc coating having improved qualities in that it is strongly alloyed to the ferrous metal and possesses a bright and smooth surface. These improved qualities I obtain without impairing the physical or electrical properties which are demanded of such articles as wire or strip.

The improved method herein referred to is also intended to cheapen the manufacture of the products of this art, and alsoto render the manufacture of same safer, since I do not use any poisonous solid substance which could form either poisonous dust or gas in the process of manufacture, and which could injure operators.

In accordance with the present invention, and in a preferred embodiment of the apparatus, for carrying this improved method into effect, I prepare ferrous wires or strip for zinc coating by leading same through an intimate mixture of selected chemicals, one of which shall possess a high nitrogen content, this chemical mixture being maintained in a heated condition upon amass of molten lead, so that the nitrogen bearing chemical shall become molten, and decompose, and thus provide a local atmosphere of nitrogen gas, through which atmosphere of nitrogen gas the ferrous wires or strip must pass. The mass of molten lead, and the heated chemicals which float thereon, are contained within a covered steel vessel, which is kept heated by an outside agency such as a Bunsen burner, though any of the usual sources of heating may be employed to maintain the temperature of this vessel.

The cover of the vessel referred to is so constructed as to allow for the entry and exit of continuous lengths of wire or strip, and also arranged to pass the wires or strip substantially through the molten mass of lead and the intimate mixture of heated chemicals so that there shall be close contact between the continuous lengths of ferrous metal and the contents of the vessel. The effect of this chemical heat treatment generally is to produce annealed ferrous metal which is free from inclavated fusion products such as 50 iron scale or deposited carburized matter, whilst the adhering chemical residual substance which covers the ferrous metal acts as a protective coating against atmospheric oxidation while the metal is cooling off.

This is my method of annealing and chemical- 1y heat treating ferrous metal, and it is done from one vessel, which is the first vessel employed in this process of preparing wires or strip for zinc coating.

By so preparing ferrous wires or strip, I am of opinion, that there is brought about a certain metallurgical condition within the porous mass of ferrous metal, through the ferrous metal oceluding in the passage through the mass of heated chemicals, the nitrogen gas which is evolved therein, as hereinbefore stated, and for which gas, iron in the sorbitic condition is known to have some preference, so that there is later brought about, when the ferrous metal is cleansed of residual matter, fluxed and led into contact with 7 molten zinc, a favourable alloying reaction (probably a form of dual reducing reaction) which reaction manifests itself in the characterxof the zinc coat and particularly with regard to the physical properties of same. A zinc coated ferrous wire so produced will allow of the severest of bending and twisting tests without the zinc coat peeling off or showing fractures, and it is noticeable that when bent or twisted that the zinc coat appears to flow with the metal base of iron, and that the zinc coat is of close crystalline structurev Whilst I prefer to regard the favourable alloying reaction, before referred to, as being due to the occlusion, by the ferrous metal, of the inert non-combustible nitrogen gas, with the resultant absence of any exo-thermal manifestations when the ferrous metal comes into contact with the molten zinc, I am aware that the subject can be explained from other standpoints, one being, that the prior occlusion of the inert nitrogen gas by the ferrous metal prevents the later occlusion of other gases which cause a calorific reactivity at the moment when the ferrous metal so occluded is brought into contact with molten zinc, this calorific reactivity not being favourable to ironzinc alloying. For example, hydrogen, as is well known, has no reducing action towards zinc oxides, and as a consequence hydrogen occluded in ferrous metal would affect the character 'of a zinc coat according to the amount occluded, whilst oxygen and some carbon gases, which are so occluded, will retardthe formation of ironzinc alloys, through the formation of surface deposits. 7 I am aware that when the heated ferrous metal comes into contact with the heated chemical mass that the. ferrous metal undergoes a process of de-oxidation, and that as a consequence gases such as oxygen and probably carbon gases 1 ,the thermal conductivity of the molten lead is the convenient means used for obtaining the required temperature of the ferrousmetal, in this heat treatment process, and also, that the high specific gravity of the molten lead is a convenient means of localizing the evolution of the nitrogen gas from the heated chemical mixture at the point where themoving wire or strip has reached the required temperature for this chemical heat treatment.

In carrying out the present invention, I find that I can obtain a very satisfactory zinc coating on ferrous metal, with economy of cost and time, and with safety in application, by the use of nitrolime (calcum cyanamide) which is the chemical on which I depend for my improved method. Preferably I use this chemical in combination with other chemicals which possess high melting points, such as soda' ash (sodium carbonate), and common salt (sodium chloride) or with refractory materials such as asbestos or sand, these are used to keep the chemical mixture porous and to expand same and thereby distribute the nitrogen gas throughout a greater mass. I find that a very advantageous combination of chemicals is as follows:-

. Percent Nitrolime (calcium cyanamide) 10 Soda ash (sodium carbonate) 35 Common salt (sodium chloride) 55 this is used at a temperature of about 800 centigrade, although the percentage combination of chemicals can be varied with variations of temperature.

With regard to the working temperature of the bath of molten zinc, I have found that a range of temperatures above 450 centigrade will pro-' duce satisfactory zinc coatings on ferrous wires and strips which have been chemically heat treated in the manner hereinbefore described.

I shall propose to use this method of, chemically heat treating ferrous wires and strip and like articles as an incidental preparatory treatment for zinc coating by means other than the hot dip process, for example, by the process of electro-deposition. Also, I shall propose to use the chemical heat treatment referred to herein, as the preparatory means of coating ferrous metals with alloys of zinc, and by any process of deposition to which this method can be applied.

The invention will be further described with reference to the accompanying sheet of explanatory drawing, which illustrates diagrammatically, and by way of example, one embodiment of apparatus for carrying out the invention, and in which:

Fig. 1 illustrates the vessel with cover, which I use to chemically heat treat ferrous metal such as wire or strip in continuous lengths.

Fig. 2 is a longitudinal section of the apparatus illustrating the arrangement of chemical heat treatment bath and associated preparatory and molten zinc baths and also the winding mechanism.

Referring now to the drawing, in Fig. 1, A represents the vessel which contains the molten lead and the heated chemical mixture, B is the cover for this vessel. The cover is brought into a secure position by means of stretcher bars fixed across the top of the cover and through the eye brackets C. The skeleton lines D show the guide bars. E shows the flue of the cover the gap of which is closed by a suitably fitted loose plate chemical mixture floating on molten lead, then the wire or strip in a cooled condition is passed into an acid bath I, for the purpose of removing the adhering chemical residual matter. Acid action is then stopped by passing the metal through a water bath- J, and later the metal is traversed through a fiuxing bath K, the flux employed being basic in character, and finally the metal is passed through the bath of molten zinc L, from which it emerges to be coiled at the winding frame M, by means of the revolving block N. The continuous lengths of wire or strip are directed from vessel to vessel and throughout the system by the guides O.

I am aware that it has heretofore been proposed to prepare-ferrous metal articles for annealing and zinc coating by treatment with ammonia gas and also by the fusing of commercial cyanides in combination with other chemicals, in such proportions and at such temperatures as will produce either a carburized or a non-carburized effect upon the ferrous metal. Also, I am aware that it has heretofore been proposed to nitrogenize ferrous metal by heating same in contact with calcium cyanamide so as to obtain .a case-hardening effect thereon. I therefore do 

